The invention described herein relates generally to structures for measuring air-cleaning filter efficiency and more particularly to structures for measuring the decontamination factors of multiple filter air-cleaning systems.
The protection provided by an air-cleaning filter is expressed as the filter's decontamination factor. If a filter stops all but one out of one thousand incident particles, the filter's decontamination factor is 1.0.times.10.sup.3. It was once thought that a two-stage filter would have an overall decontamination factor equal to the product of the decontamination factors of the individual filters. However, filtration efficiency is a function of particle size and the aerosol that penetrates the first filter may be restricted to a narrow size range from the challenge aerosol and hence be more likely to also penetrate the second filter than particles of other sizes.
Current test methods test each filter in a dual-filter system separately and may overestimate the overall decontamination factor of the dual-filter system. Moreover, these procedures for testing each filter separately require closing dampers to the process area and shutting down the process for about one hour. Temporary ducts are installed so that each filter can be tested separately. It is highly desirable to reduce the system shut-down time during testing and improve the accuracy of decontamination factor measurements.